The present invention relates to a reflective type liquid crystal display, more particularly to a reflective-type fringe field switching mode LCD(hereinafter, reflective FFS-LCD) capable of improving a reflection rate thereof.
A twisted nematic(TN) mode LCD having nematic liquid crystal compositions of positive dielectric anisotropy has been used for the conventional reflective type LCD. This reflective type TN-LCD has a low power consumption property and is used for relatively small size LCDs of an electronic clock, a digital clock and so on. However, the reflective TN-LCD has chronic problems of a poor viewing angle and a low contrast ratio.
Consequently, a reflective FFS-LCD device is now in the process of research and development to ensure a good viewing angle property, a high reflective rate and an aperture ratio. The composition of this conventional reflective FFS-LCD device is approximately illustrated in FIGS. 1 and 2.
Referring to FIGS. 1 and 2, a lower substrate 10 is opposed to an upper substrate 15 at a selected distance. A liquid crystal layer 17 having a plurality of liquid crystal molecules is interposed between the lower substrate 10 and the upper substrate 15. A counter electrode 11a and a pixel electrode 11b forming a fringe field to operate the liquid crystal molecules, are disposed on the inside surface of the lower substrate 10. A color filter(not shown) is disposed on the inside surface of the upper substrate 15. A first homogeneous alignment layer 12 is interposed between the lower substrate 10 including the counter electrode 11a and the pixel electrode 11b, and the liquid crystal layer 17. A second homogeneous alignment layer 16 is interposed between the upper substrate 15 including the color filter and the liquid crystal layer 17. At this time, the first and the second homogeneous alignment layers 12, 16 have rubbing axes R1,R2 respectively, and the rubbing axis R1 of the first homogeneous alignment layer 12 and the rubbing axis R2 of the second homogeneous alignment layer 16 form 180xc2x0, i.e. anti-parallel to each other. In addition, the rubbing axis R1 forms a selected angle with a line f which on the substrate surface, projects the fringe field that is formed between the counter electrode 11a and the pixel electrode 11b. A polarizer 8 is attached on the outside surface of the upper substrate 5 so that polarizing axis 8a thereof is equal to the rubbing axis R1 of the first homogeneous alignment layer 12. A xcex/4 plate 19 polarizing an incident light or a reflected light by xcex/4 is disposed on the outside surface of the lower substrate 10. On the outside of the xcex/4 plate 19, a reflective plate 20 reflecting the light which passes through the xcex/4 plate 19, is disposed. At this time, the xcex/4 plate 19 is disposed so that a fast (or slow) axis thereof is at an angle of 45xc2x0 with the first rubbing axis (R1).
Such conventional reflective-type FFS-LCD operates as following.
First, referring to FIG. 1, when voltage difference does not occur between the counter electrode 11a and the pixel electrode 11b, the liquid crystal molecules (not shown) are arranged so that the rubbing axes R1, R2 and the long axes thereof are parallel. Consequently, a natural light 22a becomes an incident light 22b proceeding to the same direction as a polarization axis 18a by passing through a polarizer 18. Thereafter, the direction of the incident light 22b is not changed while passing through the liquid crystal layer 17 which the rubbing axes R1, R2 and the long axes of the liquid crystal molecules are arranged side by side thereon. The incident light 22b which has passed through the liquid crystal layer 17, is at an angle of 45xc2x0 with the fast (or slow) axis of the xcex/4 plate 19, thereby becoming a right-circularly polarized light 22c passing through the xcex/4 plate 19. The right-circularly polarized light 22c is reflected b a reflective plate 20, thereby becoming a left-circularly polarized reflected light 23a. 
The reflected light 23a becomes the reflected light 23a proceeding to a crossing direction with the axis of the polarized light 18a, passing through the xcex/4 plate 19 having the fast (or slow) axis at an angle of 45xc2x0 with the proceeding direction thereof. The proceeding direction of the reflected light 23b which has passed through the xcex/4 plate 19, is orthogonal to the long axis of the liquid crystal layer 17, thereby passing through the liquid crystal layer 17 without being changed. The reflected light 22b which has passed the liquid crystal layer 17, is orthogonal to the polarizing axis 18a, thereby not passing through the polarizer 18. Accordingly, a screen becomes dark.
Next, like FIG. 2, when a fringe field F is formed between the counter electrode 11a and the pixel electrode 11b, the liquid crystal molecules (not shown) are twisted into the fringe field form. Consequently, the optical axes of the liquid crystal molecules (not shown) are at a selected angle with the polarizing axis 18a. A natural light 25a passes through the polarizer 18, thereby becoming an incident light 25b proceeding to the same direction as the polarizing axis 18a. Thereafter, the incident light 25b is at an angle of 45xc2x0 with a long axis of a liquid crystal molecule which is arranged in a fringe field F form. Therefore, an incident light 25c which has passed through the liquid crystal layer 17, becomes the incident light 25c which is at an angle of 45xc2x0 with the polarizing axis 18a. Here, the incident light 25c which has passed through the liquid crystal layer 17 coincides with the fast(or slow) axis 19a of the xcex/4 plate 19, thereby passing through the xcex/4 plate 19 without change in the proceeding direction thereof. The incident light 25c which has passed through the xcex/4 plate 19, is reflected by the reflective plate 20, thereby becoming a reflected light 26a. 
The proceeding direction of the reflected light 26a coincides with the fast (or slow) axis of the xcex/4 plate 19, thereby passing through the xcex/4 plate 19 without changing the proceeding direction thereof. The proceeding direction of the reflected light 26a which has passed through the xcex/4 plate 19, is at an angle of 45xc2x0 with the long axis of the liquid crystal molecule on the liquid crystal layer 17 and therefore the proceeding direction of a reflected light which has passed through the liquid crystal layer 17, coincides with the polarizing axis 18a. Therefore, a screen is in a white state.
Conventional reflective type liquid crystal display device has not required a backlight as light source and optical component such as the xcex/4 plate 19 has been added to the outside of a substrate thereof to improve a contrast.
However, the manufacturing cost is increased as the optical component such as the xcex/4 plate is added. Moreover, the xcex/4 plate absorbs some of an incident light or a reflected light, thereby deteriorating transmissivity of the LCD, i.e. the reflectance.
Accordingly, an object of this invention is to provide a reflective type liquid crystal display device having a good contrast ratio and a good reflectance without an additional optical component.
To accomplish the aforementioned object of this invention, the present invention according to a first embodiment provides a reflective type FFS-LCD including: a liquid crystal layer having a plurality of the liquid crystal molecules; a first substrate disposed on one side of the liquid crystal layer and in which a counter electrode and a pixel electrode, both for generating a fringe field to drive the liquid crystal molecules are formed; a second substrate disposed on the other side of the liquid crystal layer; a first homogeneous alignment layer interposed between the liquid crystal layer and the first substrate and having a rubbing axis in a selected direction; a second homogeneous alignment layer interposed between the liquid crystal layer and the second substrate, and having a rubbing axis in a selected direction; a polarizer disposed on an out side of one of the first substrate and the second substrate, and having a selected polarizing axis; and a reflective plate disposed on an out side of the other of the first substrate and the second substrate, wherein retardation of the liquid crystal layer is (2n+1)xcex/4 (here, xcex is wave of light and n is a positive number).
The present invention according to another embodiment, also provides a reflective type FFS-LCD including a liquid crystal layer having a plurality of liquid crystal molecules; a first substrate disposed on one side of the liquid crystal layer and on which a counter electrode and a pixel electrode, both for generating a fringe field to drive the liquid crystal molecules are formed; a second substrate disposed on the other side of the liquid crystal layer; a first homogeneous alignment layer interposed between the liquid crystal layer and the first substrate and having a rubbing axis in a selected direction; a second homogeneous alignment layer interposed between the liquid crystal layer and the second substrate and having a rubbing axis in a selected direction anti-parallel to the rubbing axis of the first homogeneous alignment layer; a polarizer disposed on an out side of one of the first substrate and the second substrate, and having a selected polarizing axis; and a reflective plate disposed on an out side of the other substrate of the first substrate and the second substrate, wherein the rubbing axes of the first and the second alignment layers are at an angle of 10 to 85xc2x0 with a substrate projection line of the fringe field, wherein retardation of the liquid crystal layer is (2n+1)xcex/4 (here, xcex is wave of light and n is a positive number).